Dielectric windows



June 27, 1961 J SHELTQN, JR 2,990,526

DIELECTRIC WINDOWS Filed March 2, 1953 F75, 9 FIG, /0

VENTOI? EARL J.- SHELTON, J/e.

United States Patent a DIELECTRIC WINDOWS Earl J. Shelton, Jr., Natick,Mass., asslgnor to Raytheon Company, a corporation of Delaware FiledMar. 2, 1953, 'Ser. No. 339,555 10 Claims. (Cl. 333-98) This inventionpertains to a novel window for use in microwave transmission lines, andmore particularly relates to a fluid-cooled transmission linewindow-having a thickness equal to an integral number of half wavelengths at the operating frequency.

The use of vacuum-tight dielectric windows for sealing a portion of atransmission line, such as in wave guide outputs for magnetrons, is wellknown. In previous design, such windows were made as thin as possible inorder to present a minimum reflection of high-frequency energy to betransmitted through the window. At frequencies above 7500 megacycles,the thinnest window possible is an appreciable part of an electricalwave length, and matching through the window, so as to minimizereflection, becomes arduous.

The minimum thickness of such windows is determine by mechanicalstrength and ability to maintain a vacuum. At best, windows used at highfrequency absorb considerable energy and, consequently, an undesirableamount of heat may be generated; this, in turn, reduces the power ratingof the equipment. Because of the thin windows previously used, effectivecooling was impracticable.

By increasing the thickness of the window to an integral number of halfwave lengths at the mean operating frequency, the window acts as aseries-tuned circuit in the transmission network, or a transformer withan impedance transformation of one, and produces substantially noreflection at the frequency to which it is tuned.

Furthermore, with a window of increased thickness, it is mechanicallyfeasible to introduce means into the window for cooling the same.

The cooling of the window is accomplished by introducing a plurality ofthermally-conductive elements in the window. In one embodiment, a seriesof heat-conducting wires or rods are inserted through the windowtransversely to the electric field of the high-frequency wave passingthrough the window. These rods, which make contact with the boundary ofthe transmission line, conduct away heat from the interior of the windowto said boundary and thence to a heat sink such as the ambientatmosphere surrounding said boundary.

In another embodiment, a fluid type jacket, which may include theboundary of said transmission line, surrounds the line in the vicinityof the window. A series of tubular recesses or ducts is formed in saidwindow normal to the electric lines of force. A coolant is allowed toflow through the jacket and the ducts, thereby removing heat generatedwithin the window.

FIG. 1 is a diagram depicting the electrical field configuration for the'IE mode in a cylindrical wave guide;

FIG. 2. is a plan view, partly in section, of a first embodiment of acooled dielectric window mounted within a cylindrical wave guide;

FIG. 3 is a section view taken along line 3-3 of FIG. 2;

FIG. 4 is a fragmentary longitudinal view of the wave guide and windowof FIG. 2;

FIG. 5 illustrates a second embodiment of a dielectric window which isfluid cooled;

FIG. 6 is a section view taken along line 6-6 of FIG. 5;

FIG. 8 is a plan View of a third type of dielectric Patented June 27,1961 2 window mounted within a cylindrical wave guide operating in themode illustrated in FIG. 7;

FIG. -9 represents the electrical field distribution for the TE mode ina rectangular wave guide;

FIG. 10 is a view illustrating the dielectric window similar to thatshown in FIG. 2 and mounted in a rec t-angular wave guide; and

FIG. 11 is a section view taken along line 11-].'-1 of FIG. 10.

In FIG. 1, the configuration of the electric lines of force 11 within awave guide 12 of circular cross section is shown for the TE mode. Thismode is often used since it has the lowest cutoif frequency of all TMand TE waves in a cylindrical guide and this mode can be used in asmaller tube for the same frequency.

As shown in FIG. 1, the diametric line of electric force 11' connectingpoints on the periphery of the wave guide of equal potential issubstantially linear, but the lines of force become increasingly curvedas the distance from said diametric line increases. For purposes of thisinvention, however, the lines of electric force may all be consideredlinear.

Referring to FIGS. 2 to 4, a discoidal ceramic window 14, which ispreferably any number of half wave lengths thick at the mean operatingfrequency, is mounted within the wave guide 12 substantially normal tothe direction of energy flow within the guide. The window is preferablymade from an alumina type ceramic although other types of ceramics maybe used successfully. Although the window is referred to as made ofceramic, it is possible to utilize any material which is transparent tomicrowaves and which may be firmly sealed to the wall of the wave guide.The ceramic window shown in FIGS. 2 and 3 is sealed to the inner wall ofguide 12 by any of the wellknown ceramic-to-metal sealing techniques.Illustrative methods of sealing ceramics to metal may be found inchapter 16 of Materials Technology for Electron Tubes, by Kohl,published in 1951 by Reinhold Publishing Corporation. Since theinvention does not reside in the sealing technique, a description of themanner of bonding the ceramic to the metal guide appears unnecessary.

The window 14 contains a plurality of thermally-conductive wires or rods15 which may be molded into the ceramicor inserted into tubular ductsdrilled into the edge of the ceramic disc. The rods should be made of amaterial having a high thermal conductivity, such as silver, copper, oraluminum. As shown in FIG. 2, the rods also pass through apertures 16 inthe wall of the wave guide; however, as shown in FIG. 8 (to be describedlater) it is not necessary that the guide wall be apertured. It isnecessary only that a good thermal contact be made 'between the ends ofthe rods and the wave guide wall. If the drilling technique is to beused and if the rods are to pass through the wall of the guide, theceramic disc is preferably first bonded to the guide and then the entireassembly drilled.

The rods are arranged within the window substantially perpendicular tothe electric lines of force. As previously stated, the electric lines offorce for the TE mode (shown in FIG. 1) are somewhat curved. It has beenfound in practice that the lines may be considered as parallel to thediametric line of force 111' and that the rods may be made linear andparallel to one another. It is possible, of course, to so position therods that they are everywhere normal to the electric lines of force. Inthis case, slightly curved rods would be used in the ceramic disc.

This refinement is only required in unusual cases where extremely highaccuracy is required.

Window 14is designed to maintain a vacuum in the 7 section of the linein FIG. 4 to the left of the window is adapted to be coupled to amagnetron or other evacuated high-frequency generator (not shown) Thewindow thus serves to maintain the vacuumjin the generator while[effectively transmitting a considerableportion of thefgenerated powerto an output circuit (not shown) which is subiected to ordinarypressures. V

Although the method of cooling a ceramic window by thermally-conductiverods is efiectivje, the cooling maybe improved byra fluid-cooledarrangement, such as shown in FIGS. 5 and 6. The ceramic window 14 ismounted within the cylindrical wave guide 12, as in the case of theassembly, shown in FIGS. 2 to 4. The ceramic window is provided with aseries of tubular'r'e'cesses or ducts 18 which may be formed'inaccordance with methods previously described. The wallo'r boundary ofthe wave guide also contains'aperturesjwhich are in alignment with therecesses in the window, as shown in FIG. 5. Surrounding a wave guide 12in the region of the window is a housing 20 which cooperates With-thewall of the wave guide 12 to form a fluid-tight jacket 22." The housingshown in FIGS. 5 and 6 is cylindrical and has three mutuallyperpendicular sizes, 24, 2 5, and 26, two 'of which are soldered orothe'rwise'connected to the wall, of the guide (see FIG. 5). The space30 between the'housing 20 and the wave guide 1 2. is adapted to receivea cooling fluid which flows in through arr-inlet port 32, throughtubular ducts 18 in the window, and out through an outlet port 33 asshown by the arrows in FIG. 5. The fluid is continually circulated bymeans of the usual pump (not shown) which is connected to the inletandpoutlet ports. ,In order to achieve satisfactory cooling,substantially all the cooling fluid should flow through duct's 18 ratherthan, along the wall of wave guide 12. The co'olingfiuid is preventedfrom flowing in a closed path through the compa'ratively large space 30(where the fluid would be'fnuch less' effective than when flowingthrough ducts 1=8') by means fof 'a pair of diametrically opposed fins31, 31 orother objects which are capable of impeding the flow of saidfluid.

The cooling arrangements previously described and illustrated areparticularly suitable for usejin 'guidesvin which propagation of the TEmode is desired, Should it be desired to propagate amode other than.this TE mode, it is necessary to alter the configuration of the array ofthermally-conductive elements within the window. FIG. 7 illustrates theinstantaneous enuriearfim configuration for the TE mode in a cylindricalwave guide 12. The electric lines of force 34'are now a series ofconcentric circles. If asubstantially'reflectionless window is required,it will be necessary to use an arrangement of thermally-conductive means'such as shown in FIG. 8 in which a series ofradial thermally-conductiveelements 35 are located in the edge of the discoidal window. Theelements of FIG.'8 are shown, for purposes of illustration, as rodssimilar to'those used in the window of FIGS. 2 and 3. These'rods mayjoin in'the center but may stop just short of the center, as shown inFIG. 8. It is possible to use a fluid-cooled arrangement for this mode,just as in the case of the TB mode, except that the cooling ducts arearranged radially. If fluid cooling is to be used for the mode shown inFIG. 7, it is necessary to provide apertures in the wave guide wall andto provide a housing of the type generally shown in FIGS. 5 and 6. Theprinciple of this invention is equally applicableto rectangular waveguides, one of which is shown in FIGS. 9 and 10. In FIG. 9, the electricfield distribution for the TE mode in rectangular wave guidei lz isshown and constitutes aplurality of parallel lines 41 paralleltotheshort dimension of the guide through the central: cross section. Thedielectric window 44 which, in this case, ,is rectangular to conform tothe wave guide boundaries, is sealed to the guide wall by aceramic-to-metal sealing technique already discussed in general. 2

The. thickness of the irrindowd l, likejthat shown in FIGS. 2 to4,"is"preferably an integral number ot half wave lengths at the normaloperating frequency of the system. The thermally-conductive elements 45,which may be rods or fluid-carrying ducts, are arranged parallel to thelong dimension'of thev :wave guide through the central cross section ornormal to the electric lines of forceforthis TE mode. p p Since theelectric lines of force for. the TE- m modewhere m is the number oftransverse'half wave patterns existing along the long dimension of theguide through the center of the cross section are linear, the samearrangement of thermally-conductive element s (wiresor fluid-carryingducts) may be used for-either the TE T15 3, TE etc. modes. r

' This invention is not limited to the particular details ofconstruction, materials and processes described, as many equivalentswill suggest themselves to those skilled in the art. For example, thewindow according to the invention is not restricted to use in waveguides operating in the modes herein shown and described but may beusedin guides propagating many modes. Moreover, the-window may be usedin waveguides of various shapes, such as coaxial wave guides orelliptical wave guides. '-It is, accordingly, desired that theappendedclaimsjbe given a broad interpretation commensurate with thescope of the invention within the art. 7

What is claimed is: I 7' 1. In a transmission line, a dielectric windowtransparent to high-frequency waves and positioned in said line normalto the direction of propagation of energy'along said line, said windowfurther containing a plurality of spaced tubular ducts arrangedsubstantially perpendicular to therelectric lines of force within saidline. V

2. In a wave guide transmission line, a dielectric window transparent tohigh-frequency waves andjpositioned within said guide normal to thedirection of propagation of energy along said'guide, saidwindowcontaining'aplurality of spaced tubular ducts arranged substantiallyperpendicular to the electric lines of force within saidguidie, the endsof said ducts being positioned in thermal contact with the boundary ofsaid guide. p I

3. In a transmission line, a dielectric windowtrans'parout tohigh-frequency waves and positioned trailsversely to the longitudinalaxis of said line, said window containing a plurality of spaced tubularducts arranged substantially perpendicular to the electric lines'lofforc'e within said guide, and means for producing a flow of a'coolaptthrough said ducts to remove the heat generated within said window.

4. In a transmission line, a dielectric window transparent tohigh-frequency waves and positioned within said line normal to thedirection of propagation jot :ene'rgy along said line, said windowhaving a thicknessequal to Within saidguide normal to the direction ofpropagation of energy along said guide, said window havingathicknessequal to 7 jorproducing a flow of acoolant thrgughsaid-ducts to removethe heat generated within said window.

6. In a wave guide transmission line, a dielectric window transparent tohigh-frequency waves and positioned within said guide normal to thedirection of propagation of energy along said guide, said window havinga thickness equal to where A is the wave length of the operatingfrequency and n is any integer, said window containing a plurality ofspaced tubular ducts arranged substantially perpendicular to theelectric lines of force within said guide and extending throughapertures in the boundary of said guide, and means for producing a flowof a coolant through said ducts to remove the heat generated within saidwindow.

7. In a wave guide transmission line, a dielectric window transparent tohigh-frequency waves and positioned said guide normal to th direction ofpropagation of energy along said guide, said window containing aplurality of spaced tubular ducts arranged substantially perpendicularto the electric lines of force within said guide, and means forproducing a flow of a coolant through said ducts to remove heatgenerated within said window.

8. In a wave guide transmission line, a dielectric window transparent tohigh-frequency waves and positioned within said guide normal to thedirection of propagation of energy along said guide, said window havinga thickness equal to where A is the wave length at the operatingfrequency and n is any integer, said Window containing a plurality ofspaced tubular ducts arranged substantially perpendicular to theelectric lines of force within said guide, and means for producing aflow of a coolant through said ducts to remove heat generated withinsaid window.

9. In a wave guide transmission line, a dielectric window transparent tohigh-frequency waves and positioned within said guide normal to thedirection of propagation of energy along said guide, said window havinga thickness equal to where A is the wave length at the operatingfrequency and n is any integer, said window containing a plurality ofspaced tubular ducts arranged substantially perpendicular to theelectric lines of force within said guide, a housing surrounding saidwave guide and combining with said guide boundary to form a fluid-tightjacket in the region adjacent said window, and means for producing aflow of a coolant through said jacket and said ducts to remove heatgenerated within said Window.

10. In a wave guide transmission line, a dielectric window transparentto high-frequency waves and positioned within said guide normal to thedirection of propagation of energy along said guide, said windowcontaining a plurality of spaced tubular ducts arranged substantiallyperpendicular to the electric lines of force within said guide, a seriesof apertures located in the boundary o f said guide and aligned withcorresponding ones of said ducts, a housing surrounding said wave guideand combining with said guide boundary to form a fluid-tight jacket inthe region adjacent said window, and means for producing a flow of acoolant through said jacket and said ducts to remove heat generatedwithin said window.

References Cited in the file of this patent UNITED STATES PATENTS2,400,777 Okress May 21, 1946 2,605,420 Iaffe July 29, 1952 2,611,867Alford Sept. 23, 1952 2,637,776 Edson May 5, 1953 2,748,351 Varnerin May29, 1956 OTHER REFERENCES Microwave Transmission Circuits, vol. 9 of theRadiation Laboratory Series, 1st edition, copyright May 21, 1948.Published by McGraw-Hill, pages 222 and 223 relied on.

